Field of the Invention
The present invention relates to an image forming apparatus which includes a rotating polygon mirror that scans a light beam onto an image bearing member, and a developing member that contacts or separates from a photosensitive drum, and more particularly, to a control technique to detect the number of rotations of the rotating polygon mirror at startup using a light beam.
Description of the Related Art
This type of image forming apparatus is disclosed in Japanese Patent No. 3290810, for example. In this image forming apparatus, when a scanning motor (drive unit) is started up, the scanning motor is forcibly rotated to cause the laser to emit for a predetermined time, and the laser beam scanned by a polygon mirror (rotating polygon mirror) is detected by a detecting unit. Then the number of rotations of the scanning motor is detected based on the period of the detection signal, and it is determined whether the detected the number of rotations reaches the predetermined number of rotations. If the number of rotations does not reach the predetermined number of rotations, the number of rotations is repeatedly detected after a predetermined time elapses. In Japanese Patent No. 3290810, the laser is forcibly emitted periodically, regardless the non-image region or image region of the photosensitive drum (image bearing member). Japanese Patent Application Publication No. H08-183198, on the other hand, discloses that when the scanning motor is started, the scanning motor is controlled by emitting the laser outside the image region (unblanking control). In other words, a period of the detection signal in the previous scanning is detected by the detecting unit, and the period of the next detection signal is estimated based on this signal. By generating the unblanking signal, the detection signal of the laser light can be acquired without continuously emitting the laser, whereby the start up of the scanning motor is controlled.